Such motors are known in numerous variants. In such motors it is important to prevent, in the rotor itself, short-circuits of the magnetic field of the permanent magnets that are used. To prevent this, low magnetic conductivity segments are used in the rotor; and mechanically load-bearing portions of the rotor, if they are made of ferromagnetically soft material, are at least locally configured to be so thin that they reach saturation during operation and then act practically like air, thus preventing or reducing a short circuit between adjacent rotor magnets.
In such rotors for polyphase motors, it is preferred to use a sinusoidal flux distribution in the air gap, i.e. a sinusoidal induced voltage (counter-EMF), and a sinusoidal drive current; in a three-phase system this results in a constant electromagnetic torque.
Experience has shown, however, that in such a motor it is difficult to generate an induced voltage that is a good approximation to the exact sinusoidal shape (or, expressed mathematically, has few harmonics); and because this is difficult, generation of a constant electromagnetic torque is also difficult.